Members of the general public who use such noisy products as iPods and other personal listening devices usually do not know when the noise or music is so loud that hearing loss may result. Professionals rely on a complicated relationship between sound levels and time of exposure to determine risk of hearing loss. But accurately measuring sound levels has required equipment costing thousands of dollars. Members of the public have not had an adequate indicator of the line between safe and unsafe exposure, since they have neither the equipment, education, or experience in the area of hearing conservation. Since many of these members of the public who listen to personal listening devices are children a low cost and clear indicator of potential loss of hearing from a dangerous loudness level is particularly important.
Humans are able to hear sounds in frequencies between 20 Hertz to 20,000 Hertz. 20 Hertz is a very low frequency rumble that people might describe as a vibration; 20,000 Hertz is a very high frequency. The 88 keys on the piano range from 27.5 Hertz to 4186 Hertz. The loudness of a sound is generally given in decibels (although there are other measures). People normally hear from about 0 dB, the threshold of hearing, to 140 dB, a level that cause immediate hearing loss.
Most hearing loss occurs over time from repeated exposure to loud noise. Fewer hearing losses occur from a single exposure. The repeated exposure type of hearing loss manifests itself as a loss of hearing ability, often of the softest sounds at a particular frequency. The threshold of hearing, the softest sounds that are audible for each frequency, increases as hearing loss progresses. Changes in this threshold can either be a temporary threshold shift (TTS) or a permanent threshold shift (PTS).
U.S. Pat. No. 3,970,785 to Meyer, “Tone Count Audiometric Computer,” filed May 13, 1975, and incorporated herein by reference, provides a hearing threshold level measuring apparatus for automatically determining testing hearing level in each ear and processing the test scores for either manual or automatic readout. A predetermined number of tone bursts varied randomly from one to four in each test sequence and automatically decreased in level. The subject is provided with a response panel containing pushbuttons labelled one through four. The subject's bearing threshold is then determined from his pushbutton responses to his correct burst tone counts.
U.S. Pat. No. 5,928,160 to Clark, “Home hearing test system and method” filed Oct. 30, 1996, and incorporated herein by reference, provides a home hearing test for use with a conventional home audio system comprising an audio player and a set of headphones connected to the audio player. The home hearing test includes an audio medium such as a compact disc playable in the audio player and containing a calibration tone recorded at a predetermined decibel level and a number of prerecorded sequences of tones. Each sequence has tones recorded at different decibel levels and decreasing by a step value. In a first sequence for obtaining a rough estimation of hearing threshold level, the tones start at 70 dB HL and decrease by 10 dB steps to 0 dB HL. In secondary sequences, tones start and end with tones from the first sequence and decrease in 2 dB steps. The home hearing system includes a calibration device for calibrating the output of the audio system to the ears of a person wearing the headphones against the predetermined decibel level of the calibration tone.
U.S. Pat. No. 6,350,243 to Johnson, “Portable hearing threshold tester” filed Dec. 29, 1999, and incorporated herein by reference, is a specialized portable hearing tester that provides pre-specified decibel levels to test the temporary hearing threshold shift of an individual. The portable hearing tester has a bone vibrator coupled to a discrete tone generator which is applied to a person's external skull. The bone vibrator is preferably gripped in the teeth. An alternative embodiment of the present invention uses a bone vibrator on the mastoid or forehead. The discrete tones generated through the bone vibrator can be heard by the person to test their current hearing threshold. Between 2 to 12 tones will be played for the person to hear. If the person cannot hear the selected tones at a specific decibel level which could be heard before, then that indicates a hearing threshold shift. Alternatively, discrete levels at one tone can be presented. The person need only count the number of distinct levels heard when the number of tones is reduced. When that number decreases it indicates a hearing threshold shift. The portable hearing tester can also include a two channel dosimeter which provides a way to eliminate a person's voice from the noise measurement.